The invention relates to a method for providing a surface of an article with a decoration or text.
Currently, coatings with special optical properties receive much interest for decorative applications. Some of the recent consumer products like cellular phones and electric shaver models, are provided with a Iriodin (Merck)(copyright) or Chromoflare(copyright) coating. The special property of these coatings is that the color of the coating depends on the viewing angle (flip-flop effects). For instance, for normally incident light the coating appears green, while for light that reflects at high polar angles the coating can appear blueish. The Iriodin particles consist of several layers with tailored optical properties (mica particles with metaloxyde layers). The coloring effects are due to wavelength dependent interference phenomena of light reflecting on the particles. These particles have the shape of flakes and can be applied on surfaces by conventional spraycoating processes.
Cholesteric liquid crystals intrinsically have a layered structure and can also be used to obtain these effects. Currently, the use of cholesteric materials in the form of flakes is investigated.
The cholesteric flakes are obtained by casting a 5-10 micron thick layer of cholesteric materials on a supporting substrate. Subsequently, the cholesteric layer is polymerized and milled to flakes. These flakes are applied to the surface by spraycoating. If the cholesteric liquid crystals and the flakes are well-aligned they will show the desired coloring effects.
The polymerization process is required to obtain a layer that is sufficiently stable for milling and subsequent processing of the flakes. However, once the cholesteric phase is polymerized the color of the flakes cannot be modified anymore by external treatments.
Recently, it was found that it is possible to make patterns in cholesteric layers and to control region-wise the color of the reflection. This can give an attractive additional feature to the decoration possibilities of cholesteric flakes. This feature makes it possible to write patterns (logos, brand names, advertising) in the layers. However, it is difficult to combine the flake technology and the patterning technology. An object of the present invention is to provide a method of producing decorations or text and the like on a surface, which makes use of certain advantages of certain liquid crystal materials without the necessity to have these to apply in the from of flakes.
In accordance with the present invention the above object is achieved by a method for providing a decoration or text or the like wherein at least a region-wise optically modified cholesteric liquid crystalline layer (xe2x80x9cdecorative layerxe2x80x9d) is transferred onto the surface of the article in a transfer operation. A transfer operation is an operation in which a decorative layer, usually provided with an adhesive layer, is arranged in adhering relationship with, or united with, an article. Mostly, the decorative layer is transferred from a carrier, leaving the carrier behind. In cases that it is useful to protect the decorative layer, the carrier, especially if it is thin, can be applied together with the decorative layer onto the article, so that it can function as a protection of the decorative layer at its side remote from the article.
Alternatively, a protective layer which is releasably disposed on the carrier and carries the decorative layer, can be transferred together with the decorative layer in the transfer operation. The operation of transferring the decorative layer from the carrier on to the article can be carried out using per se known procedures, for example, procedures involving a stroke motion, procedures involving a rolling motion, or procedures in which the corresponding foil is introduced into an injection molding mold and then plastic material is injected therebehind to form an article, for example, the front plate of an item of equipment.
An especially advantageous embodiment of the invention is characterized in that transfer of the patterned cholesteric liquid crystalline layer to a surface of a (plastic) article is carried out in an in-mold decoration process.
In-mold decoration is a technology in which decorations are provided to the product at the end of the production line. A transfer foil comprises a decorative foil supplied on a plastic carrier foil. Using injection molding equipment a (polymer) melt is firmly pressed to the transfer foil at elevated temperature. During this process the product is formed and the transfer foil is simultaneously xe2x80x9cgluedxe2x80x9d on the product. The technology is even capable of handling objects with non-even or (slightly) curved surfaces, like e.g. shaver products. The transfer technology appears to be suitable for application of cholesteric layers to products without the necessity to mill these layers to flakes. A further advantage of this process is that the colors are purer, the reflection is specular instead of diffuse and the viewing angle dependency is more pronounced. In contrast, during the milling and spraying of flakes the orientation of the flakes is not uniform and mixing of colors is observed.
An embodiment of a process for application of decorative foils is envisaged as follows:
(1) coating of a cholesteric layer on a plastic substrate;
(2) local modification of the cholesteric reflection band by UV exposure;
(3) curing of the cholesteric layer (e-beam, thermal);
(4) application of the cholesteric layer to the product by in-mold decoration.
The region-wise optical modification of cholesteric layers can be carried out in various manners.
It has been found in particular that there are methods by which the color of cholesteric liquid crystalline layers can be region-wise changed.
A first method is characterized by utilization of the thermochromism property (The reflection color of cholesteric materials can be made to differ by cross-linking at different temperatures).
A second, more preferred method, is characterized by the utilization of the photosensitive property (the periodicity of the cholesteric phase can be influenced, in particular by UV irradiation: when the periodicity matches the wavelength of light entering the cholesteric layer the light will be reflected). In particular this latter phenomenon can advantageously be used to produce decorations or text or the like in the layers. An extra advantage of this technique is that by using a patterned mask, e.g. a grey scale mask (a transparent sheet which has areas with different grey tints), the irradiation by UV light can be integral, so that it is not necessary to irradiate the cholesteric liquid crystal layer point by point, a technique which is needed in certain other decoration techniques.
To bring out the above effect a layer of a polymer material which has a cholesteric order, in which the axis of the molecular helix of the cholesterically ordered material is arranged transverse to the plane of the layer can be used. Such a layer can be region-wise optically modified by giving different areas a different molecular helix pitch. The latter can be realized e.g. if the layer of cholesterically ordered material comprises a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent. Compounds have been found in which the above conversion can be induced by exposure to UV radiation. Patterning can be realized by performing the exposure such that the irradiation dose is different for at least two different areas of the layer. After the exposure the cholesterically ordered material of the layer is polymerized and/or crosslinked to form a polymer which by means of the inventive transfer process can be transferred to a surface of an article.
Further objects, features and advantages of the invention, which also relates to an article having transferred onto a surface of if a region-wise optically modified cholesteric liquid cristalline layer, will be apparent from the following description of an embodiment of a transfer foil for use in the method according to the invention.
A transfer foil comprises a carrier or backing film which here is preferably a polyester film of a thickness of between about 5 and 100 xcexcm, preferably between about 15 and 50 xcexcm.
The film carries successively the following layers which are applied in the usual process which is known per se from the production of transfer foils:
Separation (or: release) layer:
This is e.g. a layer which softens under the effect of heat and which permit the further layers to be detached or released from the carrier film. The separation layer is generally of a thickness of at most 1 xcexcm.